Communication satellites receive and transmit radio signals from and to the surface of the Earth. Although Earth-orbiting communications satellites have been in use for many years, providing adequate cooling for the thermally sensitive electronics components onboard such satellites continues to be a problem.
There are two primary sources of heat with which a satellite's thermal systems must contend. One source is solar radiation. Solar radiation can be absorbed by thermal insulation shields or readily reflected away from the satellite by providing the satellite with a suitably reflective exterior surface. A second source of heat is the electronics onboard the satellite. The removal of electronics-generated heat is more problematic since such heat must be collected from various locations within the satellite, transported to a site at which it can be rejected from the satellite, and then radiated into space.
The smaller the satellite, the more problematic heat rejection can be. The limited size and mass of a smaller satellite naturally limits the surface area available for radiators and thermal control.
Heat pipes and phase change material (“PCM”) are two technologies that are commonly used in satellites to address thermal issues. A heat pipe is a closed chamber, typically in the form of tube, having an internal capillary structure which is filled with a working fluid. The operating-temperature range of the satellite sets the choice of working fluid; ammonia, ethane and propylene are typical choices. Heat input (i.e., from heat-generating electronics) causes the working fluid to evaporate. The evaporated fluid carries the heat towards a colder heat-output section, where heat is rejected as the fluid condenses. The rejected heat is absorbed by the cooler surfaces of the heat-output section and then radiated into space. The condensate returns to the heat input section (near to heat-generating components) by capillary forces to complete the cycle.
A PCM is used to damp transient temperature extremes by storing heat when the thermal load is high and releasing heat when the thermal load is low. The PCM absorbs heat via the latent heat of fusion; that is the PCM melts. The heat is absorbed without an appreciable temperature rise. Conversely, a radiator, heat pipe, thermal strap, or other means is used to remove this absorbed heat, wherein the PCM refreezes.
PCM modules are typically mounted near or on a heat source of interest. The amount of the PCM module's surface area that is exposed to the heat source is maximized to the extent possible. Heat storage performance is directly related to the interface area for heat transfer and the depth of the PCM in the module. As necessary, heat pipes are mounted to the PCM modules and/or heat source to transport the heat to a heat sink (e.g., radiator, etc.).